Immutable table library for Lua5.1 / LuaJIT

immutable.lua

--------------- pairs/ipairs polyfill
-- modified from:
-- https://gist.github.com/creationix/b56de2dde4021673f24242258efdcc80


do
  local function mm_exists(name, trig)
    local triggered = false
    trig(setmetatable({}, {[name] = function () triggered = true end}))
    return triggered
  end

  if not mm_exists("__pairs", pairs) then
    local next = pairs({})
    function _G.pairs(t)
      local mt = getmetatable(t)
      if mt and mt.__pairs then
        return mt.__pairs(t)
      else
        return next, t, nil
      end
    end
  end

  if not mm_exists("__ipairs", ipairs) then
    local iter = ipairs({})
    function _G.ipairs(t)
      local mt = getmetatable(t)
      if mt and mt.__ipairs then
        return mt.__ipairs(t)
      else
        return iter, t, 0
      end
    end
  end
end

------------ immutable library

local origTable_symbol = {}
local immutable = {}

local next = pairs({})
local function immutNext(t, k)
  local kn, v = next(rawget(t, origTable_symbol), k)
  return kn, immutable.freeze(v)
end

local iter = ipairs({})
local function immutIter(t, k)
  local kn, v = iter(rawget(t, origTable_symbol), k)
  return kn, immutable.freeze(v)
end

local proxy_mt = {
  __index = function (t, k)
    local v = rawget(t, origTable_symbol)[k]
    return immutable.freeze(v)
  end,
  __newindex = function (_, k, v)
    error("Attempt to set field " .. tostring(k) .. " to value " .. tostring(v))
  end,
  __pairs = function (immut)
    return immutNext, immut, nil
  end,
  __ipairs = function (immut)
    return immutIter, immut, 0
  end,
}

function immutable.freeze(t)
  if type(t) == "table" then
    return setmetatable({[origTable_symbol] = t}, proxy_mt)
  end
  return t
end

---@param t table
---@return table
function immutable.copyToMutable(t, seen)
  seen = seen or {}
  local copy = {}
  local orig = rawget(t, origTable_symbol) or t
  for k,v in pairs(orig) do
    if type(v) == "table" then
      if seen[v] then
        -- TODO: can probably fix this...
        error("Cyclical table, cannot trivially copy")
      end
      copy[k] = immutable.copyToMutable(v, seen)
      seen[v] = true
    else
      copy[k] = v
    end
  end
  return copy
end

---@param t table
---@param fn fun(t: table, ...): table
---@return table
function immutable.mutateWith(t, fn, ...)
  local mut = immutable.copyToMutable(t)
  return immutable.freeze(fn(mut, ...))
end

---@param im table
---@param ass table
function immutable.assign(im, ass)
  local mut = {}
  -- no deep copy needed, so this is pretty fast :)
  for k,v in pairs(im) do mut[k] = v end
  for k,v in pairs(ass) do mut[k] = v end
  return immutable.freeze(mut)
end

return immutable